A muzzle brake is a device that is attached to the muzzle of a firearm in order to limit the flash at the muzzle that accompanies the discharge of a round of ammunition. In many cases, the flash is of no consequence. In combat and especially at night, the flash may give away the location of the marksman.
The flash results from the ignition of the combustion gases that follow the bullet down the barrel and which may ignite on exposure to the air as they leave the muzzle. The muzzle brake may limit this secondary ignition in various ways. For example, the combustion gases can be dispersed radially to reduce their concentration and cool them to a temperature low enough so that ignition is not likely to occur. The forces of the dispersing gas may cause the muzzle end of the firearm to move in an opposing direction, making it more difficult to fire accurately when firing multiple rounds.
Muzzle brakes generally accomplish their goal by redirecting portions of the dispersing gas in various directions including rearward. While rearward redirection is effective in reducing flash, it typically has the deleterious effect of exposing the operator of the firearm to a concussive force created by the high energy of the expanding gases.
An effective muzzle brake, one that prevents ignition and does so with as little effect on the firearm and the operator, would be advantageous in a combat or sporting rifle.